dm: allow error target to replace bio-based and request-based targets

It may be useful to switch a request-based table to the "error" target.
Enhance the DM core to allow a hybrid target_type which is capable of
handling either bios (via .map) or requests (via .map_rq).

Add a request-based map function (.map_rq) to the "error" target_type;
making it DM's first hybrid target.  Train dm_table_set_type() to prefer
the mapped device's established type (request-based or bio-based).  If
the mapped device doesn't have an established type default to making the
table with the hybrid target(s) bio-based.

Tested 'dmsetup wipe_table' to work on both bio-based and request-based
devices.

Signed-off-by: Mike Snitzer <[email protected]>
Signed-off-by: Joe Jin <[email protected]>
Signed-off-by: Jun'ichi Nomura <[email protected]>
Acked-by: Mikulas Patocka <[email protected]>
Signed-off-by: Alasdair G Kergon <[email protected]>

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